This invention relates to apparatus for medical and therapeutic application and, more particularly, to a sensor for providing biofeedback signals to help patients relearn various physical functions, or to prevent complications that impede these physical functions.
Frequently, it is desirable to observe the mechanical pressure applied in performing some otherwise routine physical function, not only by a patient suffering from some illness or disability, but also from a recovering patient, who is trying to regain, or at least to improve some physical function. This need to observe the mechanical pressure applied by the patient in performing a particular function also extends to patients who require braces or other orthotic apparatus, as well as to prosthetic equipment, or artificial limbs, and assist devices of which crutches, wheelchairs and the like are typical.
Unconscious or involuntary bodily processes, for example, the pressure distribution applied by the sole of a foot to the inner sole of a shoe while walking, the action of the ankle, also while walking or, the application of bodily weight to a pair of crutches and the like, if made perceptible to the senses can in many instances be modified through conscious mental control to produce a proper gait, or to develop the correct use of crutches in order to avoid impairing those nerves that control arm movement and sensation, the brachial plexus. This treatment technique, often referred to as biofeedback, is beneficial. Unfortunately, known biofeedback techniques and equipment are unsatisfactory for a number of reasons and, as a consequence are not in general use by patients either in acute care, rehabilitation or at home in spite of the need for devices of this nature. However, the need for them is compelling.
For instance, biofeedback may provide for monitoring the weight applied to a limb ("limb load monitoring"). An important example of this limb load monitoring relates to limiting the weight that is borne by the limb when recovering from leg fracture or joint replacement. Total hip replacements alone are performed 200,000 times per year in the United States and "toe-touch" or "partial" weight bearing is almost always prescribed for the recovering patient by the surgeon. Partial weight bearing continues until healing has progressed sufficiently to allow the patient to apply full weight to the recovering member safely. In almost all rehabilitation centers, however, there is no way to assure compliance with this prescribed treatment except through observation by trained personnel. Although the true cost of excessive weight bearing in these circumstances does not seem to have been systematically studied, it is nevertheless reasonable to assume that this failure to systematically control the weight borne by a recovering limb prolongs healing time, increases the duration of hospitalization and patient dependent status. Thus, it seems that a reliable apparatus for systematically monitoring the load applied to a recovering limb could significantly reduce this apparent loss in patient recovery time and the effort required of trained therapists as well as reducing inefficient use of medical facilities and attendant expense.
In addition to the need for improved rehabilitation techniques, described above, there are a number of applications of limb load monitoring to gait training. Limb load devices would help amputees to apply weight to prosthetic limbs in a symmetrical manner as a prelude to progressive ambulation. Children with cerebral palsy might be taught to walk correctly by placing weight on the heel and stop walking with an equine, or downwardly pointing ankle. In this way, inpatient rehabilitation service time could be reduced, and safe ambulation promoted.
Other potential applications include rewarding the patient for meeting other strength goals. Illustratively, developing muscular contractions without significantly shortening the muscle fiber in the hand or knee, that is, isometric grip strength and knee strength might be improved through biofeedback technique. Further, individuals with rheumatic diseases also could greatly benefit from effective limb load monitoring.
Another group of patients, suffering from disabilities that confine them either permanently or temporarily to a wheelchair also could benefit from a practical application of biofeedback to their status. Paraplegics, persons with spina bifida and others who lack feeling or sensation below the waist would benefit from a biofeedback signal, cuing them to shift their weight on the seat of the wheelchair and reduce the possibility of sacral pressure ulcers. Still another group of patients, bound to wheelchairs, are those suffering from mental deterioration and strokes. In either instance, falling from a wheelchair is an all-too-common injury for patients of this nature and some biofeedback mechanism that could lock the wheelchair brakes before rising from the wheelchair would reduce injury and promote safety awareness.
Naturally, a great deal of effort has been applied to develop apparatus that will provide a reliable, inexpensive and accurate biofeedback device. These efforts involved a number of technologies that can be categorized, generally, as capacitive, resistive, hydraulic, pneumatic and also of a general, miscellaneous character for providing an accurate measure of the pressure applied by a patient to a surface.
Capacitive transducers for biofeedback operation have inherent benefits in monitoring the loading on limbs or on other body parts. As the weight of a patient is applied to a measuring capacitor, there is a relatively linear increase of capacitance with applied weight, which permits the use of simple, low cost electrical circuits to generate a signal that is directly proportional to the applied weight. In this way, a capacitive transducer might be developed to measure the total weight that is applied to complex, irregular surfaces. In spite of this potential, capacitive transducers have been overlooked during most of the past decade in favor of resistive devices. What has been lacking, and made capacitive transducer research for this purpose unattractive has been the lack of an acceptable dielectric.
The dielectric is the insulating middle layer that is interposed between the electrically conductive plates that comprise the capacitor. Ordinarily, the dielectric properties remain constant as a force, such as that applied by a limb, compresses the dielectric and brings the conductive plates closer together thereby causing the capacitance of the sensor to increase measurably.
Many dielectrics, however, are subject to creep, which is the long term change in the dielectric thickness after initial loading. Dielectrics also are subject to hysteresis, which is the difference in compression, or capacitance, at a given load with the load increasing, and with the load decreasing. The sensitivity of the dielectric, or the ratio of capacitance in the loaded and unloaded state, per application of unit weight also is an important parameter. The speed with which the sensor achieves steady state after weight is applied, or the dynamic response of the dielectric and the resiliency of the dielectric, which is the ability of the dielectric to retain its unloaded thickness and baseline capacitance after a long duration of static or dynamic loading also are significant factors in choosing suitable dielectrics. All of these properties are a function of the dielectric material composition and structure. Ideally, for the purpose of a biofeedback sensor, a dielectric should enjoy zero hysteresis and creep with a very high resilience, sensitivity, and dynamic response.
The Krusen Limb Load Monitor is an illustrative prior art device. This device is described in Craik, R. and Wannstedt, G., Proceedings 2nd Conf. Devices and Systems for the Disabled, 1975, 19-24. The capacitive transducer consists of three layers of copper- Mylar.RTM. laminate the outer two layers of which are separated from the inner layer each by a layer of "closed cell foam tape". The whole device, as shown in the paper, has the shape of an inner-sole. The characteristics of the dielectric, however, were not published. This device nevertheless fails to provide a fully satisfactory capacitive transducer because it lacks adaptability to orthotics (i.e., braces), assistive devices or wheelchairs and it is expensive.
Additionally, if conventional polyethylene foam tape is the dielectric that is used in this apparatus, the capacitive transducer also should be subject to significant hysteresis and creep, and would be unsatisfactory after repeated compression which would require frequent replacement.
An apparent improvement in the foregoing device, in which two layers of Neoprene sponge, laminated between three layers of copper foil is described in Miyazake, S., and Ishida A., Medical and Biological Engineering and Computing, 1984, pp. 309-316. Unfortunately, Neoprene, like polyethylene, has limited resilience to static and dynamic loading and also is unsatisfactory with repeated use.
On initial testing there was good agreement between force plate and transducer data on patients with foot shapes that were not markedly abnormal. However, higher local pressures from deformed feet caused excessive error (i.e., overestimation of weight). To reduce this error stiff plastic was built into the laminate to spread out area of force application. This modification, however, produced a device that was cumbersome, thick and stiff and which prevented the capacitive transducer from acceptable adaptation to shoes, orthotics, assist devices or wheelchairs.
A second major category of sensors for rehabilitation are resistive devices, which appear to be the most popular approach to solving the biofeedback sensor problem. In these resistive devices a change in electrical resistance is related to the force applied by the patient to effect that change.
Force Sensitive Resistors (FSR's) are marketed by Interlink Corp. They are thin (0.25 mm thick), physically flexible, inexpensive devices available in 1 cm.sup.2 squares. Externally there are two adjacent Mylar.RTM. sheets. On one Mylar sheet is a high resistance polymer or carbon film, and on the other Mylar sheet an interdigitated or interlocked pattern of open ended conductors is formed. Ruggedness appears to be good with extended use for this transducer and it may be used with simple electrical circuits.
For these reasons FSR's have been proposed for "everyday" rehabilitation pressure measurement in several applications. These include monitoring pressure under insensate feet in patients afflicted with diabetes and leprosy (M. Maalej, et. al., IEEE, 9th Conference of the Engineering in Medicine and Biology Society, 1987, pp. 1824, 1824). A finger and hand exercise device and "spot" high pressure sensor to prevent sacral pressure ulcers have also been described (Hyman, W. et. al., RESNA, 13th annual Conference, 1990, pp. 201, 202). It additionally has been proposed for motorized prosthesis control (Heckathorne, C., RESNA, 12th annual Conference, 1989, pp. 224225)
These resistive devices, however, are unsatisfactory for several reasons. Illustratively, the response of these resistive transducers is not linear but is substantially exponential. Unloaded resistance is effectively infinite, and resistance declines rapidly as weight in the range from 10 to 20 pounds is applied. Above this range, the electrical resistance approaches an asymptotic minimum value. Finally, as this applied weight exceeds 50 pounds the change in electrical resistance may be obscured by electrical background noise. In terms of noise, moreover, the device demonstrates significant hysteresis that varies from device to device and from trial to trial with the same device.
A microprocessor and, less satisfactory, an integrated circuit can be used to produce a linear response in which the electrical output is proportional to the applied pressure. To determine the weight applied to a large irregular contour, the output signals from many linearized sensors must be summed, and this use of more sensors makes accuracy less dependent on variations of contour. This solution to the problem increases the complexity of the associated electrical circuits, reduces reliability and dramatically increases current drain which decreases the battery life for portable devices. Because of battery drain alone, FSR's would be unacceptable for use in a wheelchair embodiment.
To use a minimum number of sensors, FSR's must be placed at points of maximum pressure. For instance, for limb load monitoring, sensors may be placed on the first and fifth metatarsals and the heel. Should a patient have an irregular foot contour or wear a cast, the resistive sensor placement must be customized.
Biofeedback for hand grip strength application using FSR's presents an even more serious problem, as hand size and placement are difficult to control. Thus, FSR's lack flexibility where total pressure is to be obtained over irregular or complex surfaces. In contrast, capacitive devices are not so inflexible or cause as much current drain. Indeed, FSR's and other resistive devices are best used, not in a dynamic environment between the patient and the therapeutic or prosthetic device, but in static situations in which local pressures are to be mapped and quantified over two dimensions, e.g. to identify skin areas that are at risk for breakdown and ulceration. Calculated forecasts based on resistive transducer data, may, for example, guide the prescription of custom shoes.
One such resistance device is based on the flexible force sensor developed by Polchaninoff in 1984 and described in U.S. Pat. No. 4,426,884. The electrical behavior of this sensor is similar to that of the FSR described above. Twenty of these sensors, moreover, are incorporated into the "multi-event notification system for monitoring critical pressure points on persons with diminished sensation of the feet" shown by Goforth in his 1987 U.S. Pat. No. 4,647,918. In the Goforth disclosure, resistors are placed under the heel, lateral plantar surface, metatarsal heads, and toes of the patient. The information signals from each of these resistors is transmitted to a portable microprocessor. The complexity, cost, and fragility of this apparatus make it unsatisfactory, however, for application to limb loading after orthopedic procedure or for gait training.
Tecscan Inc. (Boston, Mass.) has developed a pressure mapping system for gait analysis (Sensors, May 1991, pp. 21-25). in which 960 resistive elements are continuously sampled by a personal computer during ambulation. Graphics of excessive local pressure on the foot are displayed in real time. A similar device also has been developed by Tecscan for determining gluteal, or buttocks pressure contours. This latter system may be used for evaluating seating systems. Both of these systems, from the standpoint of a suitable biofeedback apparatus in a dynamic environment are expensive, lack durability, are fragile and complex.
Hydraulic devices easily determine total weight applied to irregular surfaces and, in this respect, are similar to capacitive devices. However, by definition these hydraulic devices measure the total weight by means of fluid pressure in a closed chamber. As a consequence, hydraulic devices tend to be large, heavy, bulky, thick or rigid. Nevertheless, hydraulic and conceptually similar pneumatic systems have been applied to limb load detection, grip strength detection, and seating pressure systems.
Two devices for providing objective and immediate limb loading data feedback are described in Sipe, U.S. Pat. No. 3,974,491 and Pfeiffer, U.S. Pat. No. 3,791,375. Both of these devices employ reservoirs below the heel and sole. Applied weight compresses these reservoirs and the force, or weight is monitored by a transducer on the patient's ankle. Apart from the need for a dedicated shoe, these transducers are difficult to put on and take off, and the height added to the shoe to accommodate these devices may impede ambulation in an individual whose walking is already impaired and possibly unsafe.
Hydraulic devices for measurement of grip strength are known as dynamometers. These devices are large and heavy and the applied force is read directly from a pressure gauge, which is difficult for the patient to read. Due to these limitations, they are not designed for use during exercise and are certainly inappropriate for the elderly or frail arthritic who would most benefit from grip strengthening.
Pneumatic devices also have been developed to measure pressures beneath bony prominences in evaluating wheelchair seats. Because these pneumatic transducers are essentially flattened, modified balloons, their size, shape and fragility make them inappropriate for permanently monitoring the weight of a patient on a wheelchair seat and they are not adaptable to the wide variety of wheelchair and wheelchair cushions now available.
Other physical phenomena have been employed, apart from the four mentioned above, to provide a measure of the physical pressure applied by a patient to some therapeutic or prosthetic device. Exemplary of this last group, the mechanical switching device, described in Gradisar, U.S. Pat. No. 3,702,999 measures limb loading. Each of two switches has two metal discs separated by an "O" ring. As weight is applied to the top disk, the "0" ring is compressed until electrical contact is established between the two discs to energize a buzzer. By adjusting a set screw on one disk, the weight at which the buzzer sounds also may be adjusted. This device is subject to mechanical problems; it is difficult to adjust the device to the "trigger" weight; and only one type of shoe (a "cast boot") may be used with this apparatus.
Other phenomena applied to biomedical force measurement include strain gauge and piezoelectric technologies. Strain gauges are made of coils of thin high-resistance wire or of metal or silicon impressions that are diffused or otherwise applied to a substrate. Because they enjoy a high level of accuracy, these transducers are good for gait research. Nevertheless, they also are fragile, expensive and consume large amounts of power. For these reasons, devices of this nature also are unsuited for the demands of daily therapeutic use. Additionally, these devices emit low power signals that require extensive amplification, a large volume, a controlled environment and are expensive.
In summary, weight bearing biofeedback apparatus and proposals that have characterized the prior art are undesirably complex, expensive, subject to failure, and lack durability. Although force sensitive resistors are promising for certain limited applications, they are substantially non-linear in electrical response and need dedicated electrical circuits that generate linear and integrated output signals that reflect the measured weight. For large, irregular surfaces an array of many FSR's must be used. This further increases the complexity and the cost for the system as well as the current drain while, at the same time decreasing durability and battery life.
Capacitive transducers, on the other hand are inherently linear and integrate the force information. Capacitive transducers further are independent of the body and transducer contours. These devices need not be customized and thus they are readily interchanged among patients.
On this basis, it seems that capacitive transducers should have the best potential for biofeedback operation except for the fact that no suitable dielectric has been identified that will permit the goals of simplicity, low cost, reliability, portability and low current drain to be achieved in a commercial device.